1. Field of the Invention
The present invention relates to a semiconductor chip fabrication method for dividing a semiconductor wafer into individual semiconductor chips (devices) along a plurality of crossing streets formed on the front side of the semiconductor wafer, wherein the devices are respectively formed in a plurality of regions partitioned by the streets and the back side of the semiconductor wafer is coated with a metal film.
2. Description of the Related Art
A plurality of devices such as ICs and LSIs are formed on the front side of a semiconductor wafer. The back side of the semiconductor wafer is ground to obtain a desired thickness of the semiconductor wafer. Thereafter, the semiconductor wafer is divided along a plurality of crossing streets (division lines) to obtain the individual semiconductor chips, which are used in various electronic equipment. In general, the semiconductor wafer is divided into the individual semiconductor chips by a dicing method for cutting the semiconductor wafer along the streets by using a cutting blade having a thickness of about 20 to 40 μm or by a laser processing method using a pulsed laser beam having a transmission wavelength to the semiconductor wafer.
In the laser processing method using a pulsed laser beam having a transmission wavelength to the semiconductor wafer, the pulsed laser beam is applied to the semiconductor wafer along the streets in the condition where a focal point of the pulsed laser beam is set inside the semiconductor wafer, thereby continuously forming a modified layer inside the semiconductor wafer along each street. By forming the modified layer along each street, the strength of the semiconductor wafer along the modified layer is reduced. Accordingly, by applying an external force to the semiconductor wafer along each street, the semiconductor wafer can be divided along each street to obtain the individual semiconductor chips (see Japanese Patent No. 3408805, for example). In the case that devices such as power transistors are formed on the front side of the semiconductor wafer, the back side of the semiconductor wafer is generally coated with a metal film such as a gold, silver, or titanium film having a thickness of several nanometers as a ground.
However, when such a semiconductor wafer having a metal film on the back side is cut by using a cutting blade of a dicing apparatus, burrs are left on the periphery of each semiconductor chip on the back side thereof because of the toughness of the metal film. Furthermore, there is another problem that metal dust or contamination may adhere to the cutting blade to cause clogging of the cutting blade, resulting in a short life of the cutting blade. On the other hand, in the laser processing method using a pulsed laser beam having a transmission wavelength to the semiconductor wafer, a modified layer cannot be formed inside the metal film because the laser beam is not transmitted through the metal film.
In general, examples of a metal processing method using a laser beam include a method of focusing a laser beam on the surface of metal to thereby melt the metal at the focal point and a method called ablation for evaporating the metal or generating a plasma. However, in both methods, processing dust or contamination called melt residues or debris is generated, so that these methods are not suitable for the fabrication of semiconductor chips required to avoid such processing dust or contamination.